It is possible to convert wind energy to electrical energy by using a wind turbine to drive the rotor of a generator, either directly or by means of a gearbox. The ac frequency that is developed at the stator terminals of the generator (the “stator voltage”) is directly proportional to the speed of rotation of the rotor. The voltage at the generator terminals also varies as a function of speed and, depending on the particular type of generator, on the flux level. For optimum energy capture, the speed of rotation of the output shaft of the wind turbine will vary according to the speed of the wind driving the turbine blades. To limit the energy capture at high wind speeds, the speed of rotation of the output shaft is controlled by altering the pitch of the turbine blades. Matching of the variable voltage and frequency of the generator to the nominally fixed voltage and frequency of the supply network can be achieved by using a power converter.
A typical wind turbine and power converter assembly is shown in FIG. 1. The power converter is used to interface between the wind turbine 2 driving a variable speed ac generator 4 and a supply network (labelled NETWORK). The wind turbine typically includes three turbine blades mounted on a rotating shaft and whose pitch can be controlled by means of a pitch actuator. A gearbox 8 is used to connect the rotating shaft to the rotor of the generator 4. In some cases, the rotating shaft can be connected directly to the generator rotor.
The terminals of the generator 4 are connected to the ac terminals of a three-phase generator bridge 10 which in normal operation operates as an active rectifier to supply power to a dc link 12. The generator bridge 10 has a conventional three-phase two-level topology with a series of semiconductor power switching devices fully controlled and regulated using a pulse width modulation (PWM) strategy. However, in practice the generator bridge 10 can have any suitable topology such as a three-level neutral point clamped topology or a multi-level topology (Foch-Maynard arrangement, for example).
The dc output voltage of the generator bridge 10 is fed to the dc terminals of a network bridge 14 which in normal operation operates as an inverter. The network bridge 14 has a similar three-phase two-level topology to the generator bridge 10 with a series of semiconductor power switching devices fully controlled and regulated using a PWM strategy. However, in practice the network bridge 14 can have any suitable topology, as discussed above for the generator bridge 10.
The generator bridge 10 is controlled by a generator bridge controller 20 and the network bridge 14 is controlled by a network bridge controller 22. Physically the control system may reside within the same hardware and be only a separation within software.
The ac output voltage of the network bridge 14 is filtered by a network filter before being supplied to the supply network via a step-up transformer 6. Protective switchgear (not shown) can be included to provide a reliable connection to the supply network and to isolate the generator system from the supply network for various operational and non-operational requirements.
Sudden changes in generator torque can cause serious mechanical oscillations in the drive train of the wind turbine. Such changes can occur during a grid fault where the inability to export power into the supply network results in a near simultaneous step reduction in generator torque, or as a result of a fault in the power converter. The magnitude of the mechanical oscillations is directly proportional to the magnitude of the step reduction in generator torque. In some cases the turbine assembly and drive train can be physically designed and engineered to withstand these mechanical oscillations without the need for any further protection. However, the mechanical oscillations can be kept within acceptable limits by allowing at least some of the power that cannot be exported into the supply network to be absorbed in a dynamic braking resistor (DBR) 16 that is connected in series with a suitable actuator 18 or switchgear (e.g. a semiconductor switching device such as a FET or IGBT which is sometimes referred to as a “chopper”) across the dc link 12. When the dc link voltage rises above a limit in response to a fault condition then the actuator 18 is controlled by a chopper controller 24 to short-circuit the dc link 12 so that the power that is exported from the generator 4 is absorbed by the DBR 16. The energy that is absorbed by the DBR 16 as a result of the fault condition is the integral of the absorbed power and is dissipated as heat. The DBR 16 can have any suitable physical construction and can be air- or water-cooled, for example.
If the DBR 16 is partially rated then only part of the generator power is absorbed by the DBR. In this situation the generator torque will still undergo a step reduction but it will be of a lower magnitude when compared to an arrangement where no DBR is provided. The magnitude of the mechanical oscillations in the drive train will therefore be correspondingly reduced. If the DBR 16 is fully rated then all of the generator power is absorbed by the DBR until such time as the generator 4 can start to export power into the supply network. Conventionally this may mean that the DBR 16 is rated to accept all of the generator power for a second a more. If the DBR 16 is fully rated then the generator torque will not undergo a step reduction and there is nothing to excite the mechanical oscillations in the drive train.
The difference in the generator torque response for an arrangement where there is no DBR and an arrangement where a fully rated DBR 16 is provided across the dc link 12 is shown in FIGS. 2A and 2B. Each Figure includes a series of seven graphs labelled (a) to (g) which show how the following operational parameters of the wind turbine and power converter assembly of FIG. 1 vary in a pu or “per unit” system during a grid fault where the grid voltage in the supply network dips to zero for one second:                Graph (a)—grid voltage (or supply voltage)        Graph (b)—speed of the generator rotor        Graph (c)—generator torque        Graph (d)—the amount of power that is exported to the supply network through the network bridge 14        Graph (e)—the amount of power that is exported from the generator 4 to the dc link 12 through the generator bridge 10        Graph (f)—the amount of generator power that is absorbed by the DBR 16        Graph (g)—the amount of energy that is absorbed by the DBR 16        
It can be seen from graphs (a) of FIGS. 2A and 2B that the grid voltage undergoes a step reduction from 1 to 0 at time t=65 s, remains at 0 for one second and recovers with a step increase from 0 to 1 at time t=66 s. Grid codes typically require the wind turbine to remain connected to the supply network during grid faults or transients. In other words, the wind turbine and power converter assembly must normally have some capacity for grid fault or low voltage ride-through. During such grid faults or transients the generator is unable to export power to the supply network. Graphs (d) of FIGS. 2A and 2B therefore show that the power that is exported to the supply network undergoes a step reduction from 1 to 0 at time t=65 s, remains at 0 for one second and, after a small surge at time t=66 s, starts to increase at a constant rate at time t=66 s once the grid voltage has recovered.
In the arrangement where there is no DBR then graph (c) of FIG. 2A shows that the generator torque undergoes a step reduction from 1 to 0 at time t=65 s, remains at 0 for one second and starts to increase at a constant rate at time t=66 s once the grid voltage has recovered. Graph (b) of FIG. 2A shows how the step reduction in generator torque at time t=65 s causes significant oscillations in the speed of the generator rotor. The inability to export power to the supply network during the grid fault also causes the speed of the generator rotor to increase to a peak speed at about time t=66.5 s before starting to gradually decrease.
Graph (e) of FIG. 2A shows that the amount of power that is exported from the generator to the dc link also undergoes a step reduction from 1 to 0 at time t=65 s, remains at 0 for one second and starts to increase at a constant rate at time t=66 s once the grid voltage has recovered.
In the arrangement where the actuator 18 is controlled at time t=65 s to short-circuit the dc link 12 so that the power that is exported from the generator 4 into the dc link through the generator bridge 10 is absorbed by the fully rated DBR 16 then graph (f) of FIG. 2B shows that the power that is absorbed by the DBR undergoes a step increase at time t=65 s. All of the generator power is absorbed by the DBR 16 until time t=66 s when the grid voltage has recovered and power can once again be exported to the supply network. At time t=66 s the power that is absorbed by the DBR 16 starts to decrease at a constant rate. Graph (g) shows the total amount of energy that is absorbed by the DBR 16. It will be readily appreciated that energy is the integral of the absorbed power shown in graph (f). Energy is absorbed at a constant rate between times t=65 s and t=66 s since the DBR 16 absorbs all of the generator power for the full duration of the grid fault. The rate at which energy is absorbed starts to reduce at time t=66 s as the power that is absorbed by the DBR 16 starts to decrease at a constant rate and graph (g) eventually shows a constant value at about t=66.4 s which represents the total amount of energy that has been absorbed by the DBR as a result of the grid fault.
Because all of the generator power is absorbed by the DBR 16 during the grid fault, the generator torque and the amount of power that is exported from the generator 4 remain substantially constant. There are no significant oscillations in the speed of the generator rotor and no gradual increase in the speed either. The use of a fully rated DBR therefore provides useful protection and avoids the problems that occur when the generator torque is allowed to undergo a step reduction. Although the graphs for a partially rated DBR are not shown, it will be readily appreciated that they will show a variation in the operational parameters of the wind turbine and power converter assembly that is somewhere between those shown FIGS. 2A and 2B. In other words, the generator torque will undergo a step reduction but the magnitude of the step reduction and the magnitude of the resulting oscillations in the speed of the generator rotor will depend on the partial rating of the DBR as compared to a nominal full rating.